1. Field of the Invention
The present invention relates to a flexible circuit board, and more particularly to a flexible circuit board comprising a flexible insulating sheet and a metal substrate on which said insulating sheet is applied by means of a thermoplastic polyimide layer.
2. Description of the Related Art
Recently, it has been required to dissipate effectively heat generated from electronic or electric devices provided on a printed circuit board in accordance with the increase in the number of electronic devices and the use of high power semiconductor devices. Therefore, in the case of mounting high power semiconductor devices at a high density or in a hybrid IC, it has been proposed to use a substrate made of ceramics. In order to connect the semiconductor elements and devices mounted on the printed circuit board to an external circuit, there have been widely used connector pins of such a type that the printed circuit board is clamped by the pins at its peripheral portion from front and rear surfaces. Such connector pins have a large mechanical strength and could hardly be damaged even if an external force is applied thereto, so that it is possible to attain a very high reliability in the electrical connection.
The packing density of the printed circuit board and the power of semiconductor devices have become higher and higher, and a substrate made of metal has been used in the print circuit board, the heat dissipating property of the metal substrate being superior to that of the ceramic substrate. In Japanese Patent Application Laid-open Publication Kokai Sho 61-226995 and Japanese Patent Publications Nos. 2-24394, 2-24395 and 2-25779, there nave been proposed several known flexible circuit boards using a metal substrate.
In the known flexible circuit board including the metal substrate, the metal substrate is exposed on the rear surface, and thus when the above mentioned connector pins of such a type that the flexible circuit board is clamped by the connector pins from the front and rear surfaces are used, all the connector pins would be short-circuited by means of the metal substrate, so that such connector pins having a large mechanical strength could not be utilized. In the known flexible circuit boards, there have been used surface mounting type pins.
FIG. 1 is a cross sectional view showing an example of the known flexible circuit board in which the surface mounting type pins are used. On a metal substrate 1 a flexible substrate 3 is secured by means of an adhesive layer 2. The flexible substrate 3 comprises a flexible insulating sheet 4 and a wiring pattern 5 having a predetermined configuration and formed on one surface of the metal sheet. An edge of the wiring pattern 5 is retarded inwardly from an edge of the flexible substrate 3 so that a distance between the metal pattern 5 and the metal substrate 1 measured along the surface of the flexible circuit board can be increased. On a surface of the wiring pattern 5 near its edge, a tip of a pin 6 is secured by soldering. In the known flexible circuit board including the metal substrate, the metal substrate 1 and the flexible substrate 3 is secured to each other by means of the adhesive agent layer such as an epoxy type adhesive agent. Such an epoxy type adhesive agent has a problem of heat resistance and the flexible circuit board might be peeled off the metal substrate 1 during the wire bonding for connecting bare semiconductor chips to the wiring pattern 5 or during the usage of the flexible circuit board under a high temperature. In order to remove such a drawback, in Japanese Patent Application Laid-open Publication Kokai Sho 61-226995, it has been proposed to adhere the flexible circuit substrate to the metal substrate by means of a thermoplastic polyimide film which has a self-adhesive property.
FIG. 2 is a cross sectional view illustrating another example of the known flexible circuit board having a metal substrate. In this example, a tip of a pin 6 is extended along a surface of a wiring pattern 5 of a flexible substrate 3 which is secured to a metal substrate 1 by means of an adhesive layer 2, and is connected to the wiring pattern 5 by soldering.
In the known flexible circuit board, the pin of the surface mounting type is connected to the surface of the wiring pattern by soldering. During the mounting operation of the electronic devices such as chip devices and semiconductor bear chips onto the flexible circuit board or after the mounting operation, when the external force is applied to the pin, the flexible substrate might be peeled off the metal substrate and the reliability of the connection is lost. In the known flexible circuit board shown in FIG. 2, since the pin is connected to the wiring pattern over a relatively long length so that the concentration of the external force is mitigated. But in this case, the distance between the pin and the metal substrate measured along the surface becomes short and the pins might be short-circuited via the metal substrate.
Moreover, in the known flexible circuit board having the metal substrate, the metal substrate and flexible substrate have the same configuration and are adhered to each other over their entire surfaces. Therefore, the freedom in bending is limited by the metal substrate and the application of the flexible circuit board is restricted. Further, the known flexible circuit board is connected to an external circuit by means of the surface mounting type pins, and thus a plurality of flexible circuit boards could not be easily stacked one on the other to form a laminated unit body.